1. Technical Field
The present invention relates to a projection zoom lens that is appropriately incorporated into a projector that enlarges and projects an image formed on an image display device.
2. Related Art
An optical system for a projector that enlarges and projects an image formed on an image display device needs to have (1) a long back focal length that allows a prism for combining light fluxes from three liquid crystal panels for red, green, and blue color components to be disposed, (2) a satisfactory telecentric characteristic that prevents color unevenness from occurring, and (3) a small f-number, that is, a bright optical system that allows light from an illumination system to be efficiently introduced. In recent years, the optical system also needs in many cases to have (4) not only a zooming capability but also a relatively wide angle of view at the same time that allows the projector to be readily installed in a narrow space.
To provide a projection zoom lens having a wide angle of view as well as a long back focal length, a retrofocus-type lens configuration is used in many cases, and a lens group having large negative power is typically disposed on the enlargement side.
To ensure high resolution and a flat image plane across a wide projection range of a projection zoom lens, a lens group on the enlargement side and a lens group on the reductiont side are fixed and a lens group disposed between the two lens groups is moved to change the magnification in zooming operation, and the lens group on the enlargement side is moved to bring a subject into focus in focusing operation in many cases.
The focusing group having large negative power and disposed on the enlargement side as described above is formed of a plurality of lenses having large power factors and has a relatively large size in order to provide a wide angle of view. The focusing group is therefore much larger and heavier than other lens groups.
To move the focusing group forward and backward at the time of focusing, it is typical to connect a frame of the focusing group to a primary lens barrel by using a helicoid screw structure formed along a fixed frame of the primary lens barrel and move the frame of the focusing group forward and backward through rotation. To reduce the overall size of the lens, it is necessary to dispose a helicoid screw portion on the low magnification side with respect to the focusing group. When the focusing group has a long total length and a large weight, however, the focusing group is inclined due to the weight thereof unless a helicoid screw connecting portion is long, resulting in degradation in performance of the lens. In view of the fact described above, it is preferable to minimize the total length of the focusing group and minimize the number of lenses that form the focusing group to reduce the overall weight thereof.
Since it is preferable to reduce the size and weight of the focusing group as described above, a first lens group that works as the focusing group is simplified by forming the first lens group with a single lens in related art examples (see JP-A-2001-311872 and JP-A-2002-72094). The lens described in JP-A-2001-311872 is, however, only as bright as a lens having an f-number of about 2 and only has a half angle of view of about 24°. On the other hand, the lens described in JP-A-2002-72094 is satisfactorily as bright as a lens having an f-number of about 1.7 but still only has a half angle of view of about 24°. As described above, a first lens group formed of a single lens disadvantageously cannot provide a wide angle of view.
On the other hand, there are projection zoom lenses having a half angle of view of about 30° while having a simple configuration and a relatively small number of components (see JP-A-2003-202498 and JP-A-2010-190939). In the lens described in JP-A-2003-202498 having a first lens group formed of two lenses and providing at least a half angle of view of 30°, the focusing group is formed of two glass lenses and is hence disadvantageously inclined by the weight of the glass material. In this case, the performance of the lens is believed to be degraded. To prevent the focusing group from being inclined, it is necessary to maintain sufficient strength of a portion where the focusing group is connected to a primary lens barrel, resulting in an increase in the overall size of the lens. In JP-A-2010-190939, in which a first lens group is formed of two lenses as in JP-A-2003-202498 described above, the weight of the focusing group is reduced by using a resin lens as the lens disposed on the low magnification side in the first lens group. There is, however, still room for improvement because the total length of the focusing group needs to be two lenses long.
To reduce the size and weight of a first lens group while maintaining a wide angle of view, it is conceivable to divide the first lens group into two across which negative power is distributed. That is, for example, the power of a negative lens group on the high magnification side is distributed across first and second negative lens groups (JP-A-2006-039034).
However, in JP-A-2006-039034, in which negative power on the high magnification side is distributed across a five-group configuration formed of negative, negative, positive, positive, and positive lens groups, a first lens group is formed of four lenses, which does not allow satisfactory reduction in size and weight of the first lens group as the focusing group.
As described above, to provide a wide angle of view of at least 30°, it is necessary to use a first lens group formed of at least two lenses, which means that a portion where a frame of the first lens group, which works as the focusing group, fits in a fixed frame of a primary lens barrel needs to be strong enough to support the weight of the lenses. It is therefore desired to improve known lenses in terms of reduction in size, weight, and cost of the lenses.